The present invention relates to a noise suppressing transmitter for outputting transmission signals after ambient noises are suppressed, which is used for a television telephone system or a portable telephone set.
Conventionally, as a hands-free device for a television telephone system, a noise suppressing transmitter for outputting transmission signals after ambient noises have been suppressed has been used. This noise suppressing transmitter has also been used for a portable telephone set. As an example of such a noise suppressing transmitter, one disclosed in JP-A-110349/1987 is known.
FIG. 9 is a circuit diagram showing main constituting portions of a conventional noise suppressing transmitter. In the conventional example of FIG. 9, two microphones 1 and 2 are disposed on a straight line identical to the transmitting direction of a person M. A transmission (output) signal S11 of the microphone 1 is inputted to a subtracter 3, and a transmission (output) signal S12 of the microphone 2 is phase-inverted and then inputted to the subtracter 3. The subtracter 3 extracts a transmission signal and outputs a transmission signal S13 obtained by suppressing ambient noises.
The following problems can be pointed out for the above-noted conventional noise suppressing transmitter.
(1) Distortion occurs in the output transmission signal S13 because of addition of the two signals S11 and S12. For performing such addition based on digital signal processing, the following expression (1) is established, in which the transmission signal S11 of the microphone 1 is represented by S.sub.1 (z) and the transmission signal S12 of the microphone S is represented by S.sub.2 (z).
Expression 1 EQU S.sub.2 (z)=S.sub.1 (z)D(z) (1)
D(z) in the expression (1) represents a transmission function between the two microphones 1 and 2. Based on a difference between the transmission signals S11 and S12 of the two microphones 1 and 2, S(z) of the output transmission signal S13 is represented by the following expression (3). ##EQU1##
From the expression (3), the output transmission signal S13 takes a value obtained by multiplying a signal at the time of collecting sounds only by one of the microphones 1 and 2 by the transmission function of 1-D(z). A distance between the two microphones 1 and 2 takes an interval equivalent to one sampling cycle of digital signals. For example, if a sampling frequency is 8 kHz, the distance is 4.35 cm based on the following expression (4), and the output transmission signal S13 is changed to a sound quality after having been passed through a high-pass filter (HPF).
Expression 4 EQU 1-D(z)=1-z.sup.-1 (4)
(2) For the microphone of an interphone or the like hand-set or fixed in the wall following miniaturization and making thin of a portable telephone set or a cordless telephone set, it is difficult to dispose the two microphones 1 and 2 in the direction of a straight line from a transmission voice source by keeping a proper distance because of structural reasons.
As described above, in the noise suppressing transmitter of the conventional example, distortion occurs in the output transmission signals. In addition, the two microphones cannot be disposed by keeping a proper distance in the direction of a straight line with respect to a transmission direction in the miniaturized and thin device. Consequently, it is impossible to obtain high-quality speech voices in which noises are surely suppressed. In other words, the noise suppressing transmitter of the conventional example is disadvantageous in that it cannot be installed easily in the miniaturized and thin device.
The present invention was made in order to solve the above-described problems-inherent in the prior art. A purpose of the invention to provide a noise suppressing transmitter, which can prevent the occurrence of distortion in output transmission signals, provide high-quality speech voices with noises surely suppressed and be easily installed in a miniaturized and thin device.